The macromolecules which make up asymmetric type synapses in the CNS have been investigated using purified synaptic subfractions. The postsynaptic density (PSD) appears to consist primarily of proteins identical or very closely related to tubulin and neurofilaments. The external surface of the postsynaptic membrane is characterized by glycoproteins. These glycoproteins appear to play a role in the junction of the pre- and postsynaptic membranes. Experiments to identify the neurotransmitters used by the major hippocampal afferents have been carried out. Both glutamate and aspartate are released from slices of the dentate gyrus in a calcium-dependent manner stimulated by depolarization. Moreover, lesions of major afferent tracts selectively reduce this release. These data suggest that glutamate and aspartate are neurotransmitter candidates of hippocampal afferents. BIBLIOGRAPHIC REFERENCES: Cotman, C.W., Haycock, J.W., and White, W.F. Stimulus-secretion coupling processes in brain: Analysis of norepinephrine and gamma-aminobutyric acid release. J. Physiol., 1976, 254, 475-505. Cotman, C.W. Lesion-induced synaptogenesis in brain: A study of dynamic changes in neuronal membrane specializations. J. Supramolec. Str., 1976, 4, 319-327.